Old guides say its nice to combine both cooking and fishing, but in practice cooking the fish you catch tends to take a long time ( gathering materials etc), and you'll just make more money/time selling directly your fish.
To start, get the full set of level 1 fishing gear, advanced support, and catch moat carps in whatever city you're from. Use the great blue fishing database, to find out where you can catch moat carps, as not all fishing spots have them.
http://www.great-blue.jp/?lng=en
Do that till the cap ( level 11). People say to use insect paste, but I used minnow and the bite rate is perfectly fine, and you'll never lose it unless you hit enter by mistake or catch items. Cancel every thing that isn't a single exclamation mark "!". Pipiras will bite too, but you probably wont be able to catch them. For the fishing rod, I used the composite fishing rod (it never breaks!). You will lose a few fish as it's made for big fishes (three exclamation marks), but its not bad.
At that point you'll have a decent amount of money and can start investing in cooking if you want.
Then you should go catch Nebimonites ( cap 27). Don't worry about the level gap, it almost means nothing in fishing, nebs are easy to catch anyway. I recommend catching them in sea serpent grotto. Use this map
http://www.great-blue.jp/?aid=064&lng=en
and fish at the "pond under a bridge". Only a bat wanders there and it doesn't aggro ( you need sneak to go there though). Another option is the ferry, but the constant docking sucks and wastes time imo.
